arXiv subjects

Philip A. Evans

Publications and source records attributed to Philip A. Evans.

4 recordsLinked to original sources

arXiv2022

Constraints on the X-ray Luminosity Function of AGN at z=5.7-6.4 with the Extragalactic Serendipitous Swift Survey

X-ray luminosity functions (XLFs) of Active Galactic Nuclei (AGN) trace the growth and evolution of supermassive black hole populations across cosmic time, however, current XLF models are poorly constrained at redshifts of z>6, with a lack of spectroscopic constraints at these high redshifts. In this work we \redit{place limits} on the bright-end of the XLF at z=5.7-6.4 using high-redshift AGN identified within the Extragalactic Serendipitous Swift Survey (ExSeSS) catalogue. Within ExSeSS we find one serendipitously X-ray detected z>6 AGN, ATLAS J025.6821-33.4627, with an X-ray luminosity of $L_\mathrm{X}=8.47^{+3.40}_{-3.13}\times10^{44}\mathrm{erg.s^{-1}}$ and $z=6.31\pm0.03$ making it the highest redshift, spectroscopically confirmed, serendipitously X-ray detected quasar known to date. We also calculate an upper limit on the space density at higher luminosities where no additional sources are found, enabling us to place constraints on the shape of the XLF. Our results are consistent with the rapid decline in the space densities of high-luminosity AGN toward high redshift as predicted by extrapolations of existing parametric models of the XLF. We also find that our X-ray based measurements are consistent with estimates of the bolometric quasar luminosity function based on UV measurements at $z\gtrsim6$, although they require a large X-ray to bolometric correction factor at these high luminosities.

astro-ph.HE↗

arXiv2022

An analysis of the effect of data processing methods on magnetic propeller models in short GRBs

We present analysis of observational data from the Swift Burst Analyser for a sample of 15 short gamma-ray bursts with extended emission (SGRBEEs) which have been processed such that error propagation from Swift's count-rate-to-flux conversion factor is applied to the flux measurements. We apply this propagation to data presented by the Burst Analyser at 0.3-10 keV and also at 15-50 keV, and identify clear differences in the morphologies of the light-curves in the different bands. In performing this analysis with data presented at both 0.3-10 keV, at 15-50 keV, and also at a combination of both bands, we highlight the impact of extrapolating data from their native bandpasses on the light-curve. We then test these data by fitting to them a magnetar-powered model for SGRBEEs, and show that while the model is consistent with the data in both bands, the model's derived physical parameters are generally very loosely constrained when this error propagation is included and are inconsistent across the two bands. In this way, we highlight the importance of the Swift data processing methodology to the details of physical model fits to SGRBEEs.

astro-ph.HE↗

arXiv2020

A Demonstration of Extremely Low Latency $γ$-ray, X-Ray & UV Follow-Up of a Millisecond Radio Transient

We report results of a novel high-energy follow-up observation of a potential Fast Radio Burst. The radio burst was detected by VLA/realfast and followed-up by the Neil Gehrels Swift Observatory in very low latency utilizing new operational capabilities of Swift (arXiv:2005.01751), with pointed soft X-ray and UV observations beginning at T0+32 minutes, and hard X-ray/gamma-ray event data saved around T0. These observations are $>10$x faster than previous X-ray/UV follow-up of any radio transient to date. No emission is seen coincident with the FRB candidate at T0, with a 0.2s fluence $5σ$ upper limit of $1.35\times10^{-8}$ erg cm$^{-2}$ (14-195 keV) for a SGR 1935+2154-like flare, nor at T0+32 minutes down to $3σ$ upper limits of 22.18 AB mag in UVOT u band, and $3.33\times10^{-13}$ erg cm$^{-2}$ s$^{-1}$ from 0.3-10 keV for the 2 ks observation. The candidate FRB alone is not significant enough to be considered astrophysical, so this note serves as a technical demonstration. These new Swift operational capabilities will allow future FRB detections to be followed up with Swift at even lower latencies than demonstrated here: 15-20 minutes should be regularly achievable, and 5-10 minutes occasionally achievable. We encourage FRB detecting facilities to release alerts in low latency to enable this science.

astro-ph.HE↗

arXiv2015

An Analysis of Chandra Deep Follow-up GRBs: Implications for Off-Axis Jets

We present a sample of 27 GRBs with detailed Swift light curves supplemented by late time Chandra observations. To answer the missing jet-break problem in general, we develop a numerical simulation based model which can be directly fit to the data using Monte Carlo methods. Our numerical model takes into account all the factors that can shape a jet-break: (i) lateral expansion (ii) edge effects and (iii) off-axis effects. Our results provide improved fits to the light curves and constraints on physical parameters. More importantly, our results suggest that off-axis effects are important and must be included in interpretations of GRB jet breaks.

astro-ph.HE↗